client: Clean up return values in clean() paths
client: Clean up return values in clean() paths

- Have clean() return -1 on failure or 0 on success

- Warn on release errors from release(), and remove broken error test in
checkin()

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr